1. Field of the Invention
The invention relates generally to semiconductor switching devices and in particular to an improved reverse blocking triode thyristor, commonly known as a silicon controlled rectifier or SCR. The invention provides a new SCR with improved operational characteristics.
2. Related Art
Thyristors are solid state switches which may be triggered from an open circuit or high impedance state to a short circuit or low impedance state. A thyristor has four electrical control terminals, including an anode gate, a cathode gate, an anode and a cathode. In practice, typically only one of the gates is used with the anode and the cathode. When a thyristor is fabricated as part of an integrated circuit, the thyristor may include a fifth electrical terminal, the substrate. Prior to triggering, a thyristor acts as an open circuit, capable of withstanding a specified, rated voltage applied across the anode and cathode. After triggering, a thyristor provides a low-impedance current path between the anode and cathode and remains in that condition until the current flow stops or is reduced below a predetermined threshold level called the holding current. The magnitude of the holding current may be adjusted using additional circuit elements external to the thyristor. Triggering occurs by supplying current to the gate. Once the thyristor is triggered, the gate current may be removed and the thyristor will remain triggered.
"Thyristor" is a generic term which includes different specific types of devices. One type of thyristor is a reverse blocking triode thyristor, commonly called a silicon controlled rectifier or SCR. An SCR is a unidirectional device, conducting load current in only one direction. Another type of thyristor is a bidirectional triode thyristor, commonly known as a triac. A triac is a bidirectional device, conducting current in either direction in response to the gating or trigger current.
Structurally, a thyristor consists of several alternating layers or regions of doped semiconductor material forming multiple p-n junctions. For example, a thyristor is commonly made of silicon which has been doped n-type or p-type. A load is coupled to the anode and cathode, across the multiple junctions. The trigger current is applied to one of the junctions. The trigger current allows the load current to flow through the device, setting up a regenerative action which keeps the current flowing, even after the trigger current is removed.
For analysis and characterization, the thyristor is modelled as two bipolar transistors. In the model, a PNP transistor has its emitter coupled to the anode and an NPN transistor has its emitter coupled to the cathode. The base of the PNP transistor is modelled as being coupled to the collector of the NPN transistor, and the collector of the PNP transistor is modelled as being coupled to the base of the NPN transistor. However, the n-type region which forms the PNP base is the same n-type region which forms the NPN collector. Similarly, the p-type region which forms the NPN collector is the same p-type region which forms the NPN base. Modelling the thyristor as two transistors allows simpler analysis of the device.
Many thyristors, including SCRs, are used as discrete devices with gate (either anode gate or cathode gate), anode and cathode terminals in electrical contact with their respective p-type or n-type regions. In theory, both gates of the SCR may be used, but in practice, usually only one gate is used for controlling the SCR operation. The terminals may be electrically wired with other system components. Such thyristors are made by forming individual layers on the surface of a semiconductor wafer and doping the respective layers accordingly. Current flow in such thyristors essentially vertical, from layer to layer between the anode and cathode.
Other thyristors are made for use in an integrated circuit in conjunction with active and passive devices formed on the surface of the same semiconductor wafer. Such integrated circuits implement digital or analog functions, or both. The manufacturing process for producing such integrated circuits is typically optimized for production of NPN transistors. NPN transistors produced according to the process have well-controlled, nearly ideal operational parameters. Other devices have parameters which vary greatly from the ideal or which display a wide tolerance.
In many integrated circuit fabrication processes, PNP transistors display less than ideal operational characteristics. For example, in a lateral PNP transistor, the width of the base region is defined by the spacing of the p-type regions which form the collector and emitter of the transistor. This spacing is set by photolithographic considerations, and is much greater than the base width of a vertically oriented NPN transistor, which is set by diffusion or implant of the n+ emitter region into the p-type base. Base width is a key characteristic for a bipolar transistor. The common emitter current gain, or .beta., of the transistor is a strong function of base width. PNP transistors, including lateral PNP transistors, suffer from other operational limitations.
One known operational limitation associated with lateral PNP transistors is injection of current into the semiconductor substrate on which the transistor is fabricated. The substrate has a relatively high resistivity and is commonly electrically tied to the most negative potential in the circuit. Typically, the PNP transistor is surrounded with an isolation region doped p-type. The isolation region is electrically coupled to the most negative potential in the circuit to reverse bias the p-n diode formed between the p-type isolation region and adjacent n-type regions. The isolation region thereby electrically isolates the PNP transistor from the surrounding circuit. However, the isolation region operates as an additional, parasitic, collector for the PNP transistor, collecting charge emitted from the emitter and injecting that charge into the substrate.
Injection of current into the substrate is problematical for several reasons. A large portion of the PNP transistor emitter current is diverted from the true collector to the parasitic collector. As much as seventy percent of emitter current is lost into the substrate. Injection of charge into the substrate, combined with the relativity high resistivity of the substrate, may cause the potential of portions of the substrate to deviate substantially from the most negative potential in the circuit. This could cause the normally reverse biased isolation junctions to become forward biased, injecting further current into the substrate, further raising the potential of the substrate. This may result in an uncontrollable condition similar to latch up and may cause permanent destruction of the device.
Another problem created by injection of current into the substrate is diversion of this current from the circuit. If the cathode current of a thyristor is only, say, thirty percent of the anode current because the other seventy percent is substrate current, operation of the circuit which includes the thyristor may be affected. The current may be used to set other voltage levels or to trigger other devices which will not operate properly at the reduced current level.
Moreover, the magnitude of the substrate current is difficult to predict, control or compensate. Substrate current is strongly dependent on operating temperature of the circuit, which may vary widely under differing conditions. Also, substrate current is strongly dependent on manufacturing process parameters, which may vary greatly from circuit to circuit.